Instant power provided by small muscle (7)
I believe the answer is:
triceps
'muscle' is the definition.
(I know that triceps is a muscle)
'instant power provided by small' is the wordplay.
'instant' becomes 'trice' (both can mean a short amount of time).
'power' becomes 'p' (symbol in physics).
'provided by' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'trice'+'p'+'s'='TRICEPS'
(Other definitions for triceps that I've seen before include "Three-headed muscle" , "Extensor muscle" , "army feature?" , "Muscles" , "Green citrus fruit" .)